1. Field of the invention
This invention relates to an optical modulator for use in waveguides such as optical fibres or planar substrate devices.
2. Related Art
Hitherto, an optical multi-waveguide interference filter has been proposed in which a first waveguide capable of single mode transmission, is coupled to a second waveguide capable of supporting at least two transmission modes. A third waveguide supporting a single mode is used to pick up light from the second, multimode waveguide. In use, light in a single mode passes from the first waveguide into the second waveguide where it is transmitted in two modes, which interfere with one another. By an appropriate selection of the is length of the second waveguide and its optical characteristics, the interfering modes produce an amplitude peak at the entrance to the third waveguide, which shifts diametrically across the input to the third waveguide as a function of wavelength. The filter is described in detail in "An Optical Multi-waveguide Interference Filter" M. Earnshaw et al, Optics Communications 116 (1995), 339-342, May 1, 1995.
Similar two mode fibre interferometric devices have been proposed by D. Kreit et al in "Two-mode fibre interferometer/amplitude modulator" Applied Optics, December 1996, Vol 25, No 23, pp 4433-4438, which involve stretching the fibre to vary the interference between the modes, so that the optical output is a function of applied stress.
Reference is also directed to M. A. Abore et al "Analysis of the insertion loss and extinction ratio of two-mode fibre interferometric devices" which discloses an optically pumped bi-moded fibre in which an input optical signal switches the interference between the modes in the fibre, so as to switch the output.
It has also been proposed to sense temperature by sensing mode-mode interference in a birefringent optical fibre, as described by W. Eickoff, Optics Letters, Vol. 6, No. Apr. 4, 1981 pp 204-206.
Electro-optic modulation of a silica-based fibre has been proposed by T. Fujiwara et al, Electronics Letters, Mar. 30, 1995, Vol. 31 No. 7, pp 573-575. In order to pole the fibre and provide it with an electro-optic coefficient, a voltage was applied to wires inserted into the fibre whilst irradiating it with ultraviolet radiation. Thereafter, the application of a voltage to the wires produced a phase shift for optical radiation transmitted through the fibre.